Recent attempts to achieve high-altitude, long-endurance flight for aircraft have been based on two approaches. The first approach focused on a lighter-than-air, blimp-like craft having solar cells and electric motors driving multiple propellers. The second approach focused on a conventional aircraft having long, narrow conventional airfoils/wings (high aspect ratio wings) covered with solar cells and having multiple electric motors and propellers disposed along the wings.
An example of the first approach includes the High-Altitude Airship by Lockheed Martin fabricated for the Missile Defense Agency, which has the potential to be unwieldy, expensive, and susceptible to high wind loads. An example of the second approach includes the Helios craft fabricated by Aerovironment for NASA, which had fragile, 240-foot long wings. Helios crashed due to turbulence and aeroelastic instability off Hawaii in 2003.
According to the Breguet endurance formula for propeller-driven aircraft, an aircraft's endurance is proportional to η, the propeller efficiency, CL3/2/CD, where CL is the lift coefficient and CD is the drag coefficient, and √(ρS), where ρ is the air density and S is the wing area. Conventionally, if the aircraft does not carry fuel but is rather powered by solar energy, it is designed to have the lowest possible weight (Wt) and the highest possible energy storage efficiency E/Wt to enable flight during the night.
One way to increase CL3/2/CD is with high-aspect-ratio, long flimsy wings that are known to inhibit aircraft safety and maneuverability. Increasing the wing area S in a conventional wing adds too much weight, and adding battery-assisted solar cells or fuel cells makes the vehicle too heavy, which reduces payload ratio.
For these and other reasons, there is a need for the present invention.